The present invention relates to novel copolymers of alpha-olefins. More specifically, it relates to novel copolymers of ethylene with other alpha-olefins comprised of copolymer chains with compositions which are intramolecularly heterogeneous and intermolecularly homogeneous, as well as, to a process for making these copolymers and their use in lube oil and elastomer applications.
For convenience, certain terms that are repeated throughout the present specification are defined below:
a. Inter-CD defines the compositional variation, in terms of ethylene content, among polymer chains. It is expressed as the minimum deviation (analogous to a standard deviation) in terms of weight percent ethylene from the average ethylene composition for a given copolymer sample needed to include a given weight percent of the total co-polymer sample which is obtained by excluding equal weight fractions from both ends of the distribution. The deviation need not be symmetrical. When expressed as a single number for example 15% Inter-CD, it shall mean the larger of the positive or negative deviations. For example, for a Gaussian compositional distribution, 95.5% of the polymer is within 20 wt. % ethylene of the mean if the standard deviation is 10%. The Inter-CD for 95.5 wt. % of the polymer is 20 wt. % ethylene for such a sample.
b. Intra-CD is the compositional variation, in terms of ethylene, within a copolymer chain. It is expressed as the minimum difference in weight (wt.) % ethylene that exists between two portions of a single copolymer chain, each portion comprising at least 5 weight % of the chain.
c. Molecular weight distribution (MWD) is a measure of the range of molecular weights within a given copolymer sample. It is characterized in terms of at least one of the ratios of weight average to number average molecular weight, M.sub.w /M.sub.n, and Z average to weight average molecular weight, M.sub.z /M.sub.w, where: ##EQU1## Ni is the number of molecules of weight Mi.
d. Viscosity Index (V.I.) is the ability of a lubricating oil to accommodate increases in temperature with a minimum decrease in viscosity. The greater this ability, the higher the V.I.
Ethylene-propylene copolymers, particularly elastomers, are important commercial products. Two basic types of ethylene-propylene copolymers are commercially available. Ethylene-propylene copolymers (EPM) are saturated compounds requiring vulcanization with free radical generators such as organic peroxides. Ethylene-propylene terpolymers (EPDM) contain a small amount of non-conjugated diolefin, such as dicyclopentadiene; 1,4-hexadiene or ethylidene norbornene, which provides sufficient unsaturation to permit vulcanization with sulfur. Such polymers that include at least two monomers, i.e., EPM and EPDM, will hereinafter be collectively referred to as copolymers.
These copolymers have outstanding resistance to weathering, good heat aging properties and the ability to be compounded with large quantities of fillers and plasticizers resulting in low cost compounds which are particularly useful in automotive and industrial mechanical goods applications. Typical automotive uses are tire sidewalls, inner tubes, radiator and heater hose, vacuum tubing, weather stripping and sponge doorseals and Viscosity Index (V.I.) improvers for lubricating oil compositions. Typical mechanical goods uses are for appliance, industrial and garden hoses, both molded and extruded sponge parts, gaskets and seals and conveyor belt covers. These copolymers also find use in adhesives, appliance parts as in hoses and gaskets, wire and cable and plastics blending.
As can be seen from the above, based on their respective properties, EPM and EPDM find many, varied uses. It is known that the properties of such copolymers which make them useful in a particular application are, in turn, determined by their composition and structure. For example, the ultimate properties of an EPM or EPDM copolymer are determined by such factors as composition, compositional distribution, sequence distribution, molecular weight, and molecular weight distribution (MWD).
The efficiency of peroxide curing depends on composition. As the ethylene level increases, it can be shown that the "chemical" crosslinks per peroxide molecule increases. Ethylene content also influences the rheological and processing properties, because crystallinity, which acts as physical crosslinks, can be introduced. The crystallinity present at very high ethylene contents may hinder processibility and may make the cured product too "hard" at temperatures below the crystalline melting point to be useful as a rubber.
Milling behavior of EPM or EPDM copolymers varies radically with MWD. Narrow MWD copolymers crumble on a mill, whereas broad MWD materials will band under conditions encountered in normal processing operations. At the shear rates encountered in processing equipment, broader MWD copolymer has a substantially lower viscosity than narrower MWD polymer of the same weight average molecular weight or low strain rate viscosity.
Thus, there exists a continuing need for discovering polymers with unique properties and compositions. This is easily exemplified with reference to the area of V.I. improvers for lubricating oils.
A motor oil should not be too viscous at low temperatures so as to avoid serious frictional losses, facilitate cold starting and provide free oil circulation right from engine startup. On the other hand, it should not be too thin at working temperatures so as to avoid excessive engine wear and excessive oil consumption. It is most desirable to employ a lubricating oil which experiences the least viscosity-change with changes in temperature.
The ability of a lubricating oil to accommodate increases in temperature with a minimum decrease in viscosity is indicated by its Viscosity Index (V.I.). The greater this ability, the higher the V.I.
Polymeric additives have been extensively used in lubricating oil compositions to impart desirable viscosity-temperature characteristics to the compositions. For example, lubricating oil compositions which use EPM or EPDM copolymers or, more generally, ethylene-(C.sub.3 -C.sub.18) alpha-olefin copolymers, as V.I. improvers are well known. These additives are designed to modify the lubricating oil so that changes in viscosity occurring with variations in temperature are kept as small as possible. Lubricating oils containing such polymeric additives essentially maintain their viscosity at higher temperatures while at the same time maintaining desirable low viscosity fluidity at engine starting temperatures.
Two important properties (although not the only required properties as is known) of these additives relate to low temperature performance and shear stability. Low temperature performance relates to maintaining low viscosity at very low temperatures, while shear stability relates to the resistance of the polymeric additives to being broken down into smaller chains.
The present invention is drawn to a novel copolymer of ethylene and at least one other alpha-olefin monomer which copolymer is intramolecularly heterogeneous and intermolecularly homogeneous. Furthermore, the MWD of the copolymer is very narrow. It is well known that the breadth of the MWD can be characterized by the ratios of various molecular weight averages. For example, an indication of a narrow MWD in accordance with the present invention is that the ratio of weight to number average molecular weight (M.sub.w /M.sub.n) is less than 2. Alternatively, a ratio of the Z-average molecular weight to the weight average molecular weight (M.sub.z /M.sub.w) of less than 1.8 typifies a narrow MWD in accordance with the present invention. It is known that the property advantages of copolymers in accordance with the present invention are related to these ratios. Small weight fractions of material can disproportionately influence these ratios while not significantly altering the property advantages which depend on them. For instance, the presence of a small weight fraction (e.g. 2%) of low molecular weight copolymer can depress M.sub.n, and thereby raise M.sub.w /M.sub.n above 2 while maintaining M.sub.z /M.sub.w less than 1.8. Therefore, polymers, in accordance with the present invention, are characterized by having at least one of M.sub.w /M.sub.n less than 2 and M.sub.z /M.sub.w less than 1.8. The copolymer comprises chains within which the ratio of the monomers varies along the chain length. To obtain the intramolecular compositional heterogeneity and narrow MWD, the copolymers in accordance with the present invention are preferably made in a tubular reactor. It has been discovered that to produce such copolymers requires the use of numerous heretofore undisclosed method steps conducted within heretofore undisclosed preferred ranges. Accordingly, the present invention is also drawn to a method for making the novel copolymers of the present invention.
Copolymers in accordance with the present invention have been found to have improved properties in lubricating oil. Accordingly, the present invention is also drawn to a novel oil additive composition which comprises basestock mineral oil of lubricating viscosity containing an effective amount of a viscosity index improver being copolymer in accordance with the present invention.